The necessity to detect gases in the air has become increasingly important of late.
For instance, it is desirable to trigger switching processes depending on the presence of noxious substances in the air. Particularly in connection with motor-vehicle ventilation, numerous efforts have been made to act upon the ventilation system of a vehicle by means of a sensor sensitive to gas, so that when an unusually high load of noxious substances is present in the surrounding air, the fresh air supply to the vehicle is interrupted and the system is switched, for instance, to air recirculation.
It is known to detect certain gases with the aid of metal oxide gas sensor elements. The sensor element here consists mostly of a metal oxide, which is brought to a working temperature by suitable heating.
Tin dioxide sensor elements heated to a temperature of approximately 250.degree. C. to 450.degree. C. are widely used. As a rule, a substance acting as a catalyst is admixed with the tin dioxide, for instance, platinum, palladium and rhodium.
Sensor elements of this kind experience a resistance reduction in the presence of oxidizable gases and the tin dioxide releases oxygen. When the sensor is again exposed to normal air, the surface reacts again with the oxygen to form tin dioxide. Thus, the process is reversible and the sensor is not subject to wear.
It has been found that sometimes a rough difference occurs between the expected resistance value of the sensor and the concentration of oxidizable gas, e.g. carbon monoxide. More detailed research has shown now that in the simultaneous presence of reducible gases, these gases have a strong influence on the reaction between the reducible gases and the sensor surface.
In extreme cases in spite of high concentration levels, no reaction or only a minimal electric reaction will take place in the sensor. As an example, reference can be made to the reaction in the simultaneous presence of carbon monoxide (CO) and nitrogen oxide (NOx). The cause is the direct mutual reaction of the gases close to the hot sensor surface, whereby the catalytic substance is influenced.
A particular disadvantage of the aforementioned heated tin dioxide sensor is its low sensitivity with respect to diesel exhaust gases. This appears to be due to the fact that tin dioxide sensors always react with a resistance reduction in the presence of an oxidizable gaseous substance. So for instance, tin dioxide sensors react to carbon monoxide, peroxide or gasoline vapors correspondingly to bar 60 in FIG. 1. These components are present in the exhaust gases of engines running on gasoline, so that this clear reaction of the tin dioxide sensor occurs.
If this tin dioxide sensor is exposed to nitrogen oxides (NOx) in the laboratory, its inner resistance increases, as shown by bar 50 in FIG. 1.
In the exhaust gases of a Diesel engine particularly when it runs under load, the substances corresponding to bar 60 in FIG. 1 coexist with the ones corresponding to bar 50 in FIG. 1. As a result, it has been found that the tin dioxide sensor reacts less clearly than one would expect based on the measured and proven proportions of the gases which are resent. The reaction of the tin dioxide sensor corresponds to bar 70 in FIG. 1.
This is highly disturbing for the sensor operation, since when such sensors are used in practice, they are very often exposed simultaneously to oxidizable as well as reducible gases.